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Test-subject objections

So the University of Aberdeen has developed a test for schizophrenia that is 98 per cent accurate (Campus round-up, 8 November). Should I be worried if I test positive?

Around 1 per cent of the population has schizophrenia, so in a sample of 10,000 you will get 98 of the 100 with schizophrenia correctly testing positive. However, 2 per cent of the 9,900 - 198 - who haven't got it will also test positive. Therefore, the probability of a positive test being correct is 98/(98+198), or 33 per cent. So, more than two-thirds of those positively diagnosed by the test will in fact not be schizophrenic. Reassuringly high accuracy rates are meaningless unless one knows the prevalence of what is being tested.